You could paint the turret white first and then fit the template over the details. Then paint with the top coat.  I did this on my SMS Konig battleship turrets and it worked just fine.

The Italians probably hand-painted the circles so if it's a little sloppy it should be ok I'd think? 

I would use a piece of unprinted decal paper and some liquid decal film, (such as Microscale's), coat paper with decal film, let dry, use a circle template or stencil to paint white circle, coat again and let dry. Apply decal and use decal setting solution to snug it down over any protrusions.